One critical issue in integration of porous materials, such as e.g. low-k materials, during semiconductor processing is the degradation of e.g. their dielectric properties during plasma etching and/or resist stripping. The plasmas used during such processes typically comprise oxygen-containing species. The main reason for degradation of the dielectric properties of porous materials is the removal of carbon containing hydrophobic groups by using these oxygen containing plasmas.
Carbon depletion occurs when, for example, a Si—CH3 bond is broken and the carbon is replaced by a silicon-dangling bond. This carbon depletion results in the formation of silanol (Si—OH) through a variety of intermediate reactions. This leads to an increase in k-value for the damaged portion of the porous material and converts the inherently hydrophobic low-k material into a hydrophilic material. Subsequent adsorption of moisture, e.g. water, or other polar molecules having high polarizability, mediated by hydrogen bonding, can significantly increase the effective k-value of the material, e.g. to a k-value>>80. Degree and depth of plasma damage depends on the pore size and the pore connectivity of the porous material and therefore, ultra low-k materials with e.g. k-value lower than 2.6, which normally have a relatively large pore size, suffer much more from this plasma damage than micro-porous materials with a k-value of higher than 2.6. The extent of the damaged portion of the dielectric material at sidewalls of etched material is also expected to increase as the porosity of the porous material increases, and the extent of such damage on overall electrical performance gains importance as the spacing between interconnect lines shrinks. Therefore, sidewall dielectric damage has a major impact on the performance of advanced interconnects, and a reliable analysis method for evaluating the extent of such damage is desirable.
In general, the depth and profile of carbon depletion is evaluated using complicated analytical techniques like, for example, Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), Energy Filter transmission Electron microscopy (EFTEM) etc., or by using the so called HF-dip test. TOF-SIMS is a type of SIMS in which an ultra-low current incident ion beam is used and by which information regarding chemical composition of the outermost surface of solids can be obtained. EFTEM is a technique that detects the variation of the atomic concentrations of elements such as C, O and Si through the cross-section of a feature.
Using TOF-SIMS, it is assumed that only the carbon concentration is responsible for the hydrophobic properties that define the dielectric constant of the film, independent of how they are bonded and integrated in the porous material structure. TOF-SIMS data for determining the depth of low-k damage are related to the carbon concentration in the surface region of the low-k porous material and these data are compared with the carbon concentration in the bulk of the low-k material. The carbon depletion in the surface region after etching and stripping is then an indication of the low-k damage. However when such low-k materials, e.g. films, are subjected to HF dip test there is no clear correlation between carbon depletion and plasma damage.
A HF dip test is based on the fact that damaged low-k dielectric material shows a higher etch rate than undamaged material. Normally this test more directly reflects the hydrophilic properties of the porous material but an autocatalytic mechanism of interaction of HF with SiO2 makes this test not reliable because the etch rate and the calculated depth of damage depends on the HF concentration.
Carbon depletion cannot always directly be correlated with plasma damage. For example, when a highly polymerizing chemistry is used (e.g. CxFyHz plasma), the carbon depletion is compensated by the deposition of CFx polymers resulting in equal or increased carbon concentrations in the low-k porous material surface. According to traditional interpretation of TOF-SIMS results, this surface should be the most hydrophobic (no “damage”). However, HF dip tests show a larger etch rate of this sample in comparison with an undamaged low-k reference material, which in general is pristine. Therefore, the fluorocarbon polymer that is formed during the etching process and that fills the pores of the porous material is not able to provide the same hydrophobic properties as the original hydrophobization agents. These facts show the importance for the development of special measurement procedures that give more direct analysis on the degree of internal hydrophilization related to plasma damage.
In addition, all the existing methods described above and other methods available in the prior art are destructive and/or very complicated and do not give information directly correlated with loss of hydrophobicity, which has the largest effect on the dielectric properties of the porous material. Thus, existing methods to determine low-k damage have serious drawbacks and shortcomings. A method that allows for determining the depth of damage and exact hydrophobicity of low-k material is desirable. Additionally, there is a need for a simple non-destructive method for use in developing and screening different low-k materials, especially for ultra low-k materials that will be used for future technologies.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.